Siloxanes comprising nitrogen functionality, more particularly siloxanes carrying amino groups, are claiming increased importance in the textile finishing sector, and also for important leave-on applications in the cosmetics additives sector, such as, for example, hair conditioning. Not least from the standpoint of sustainability, but also that of biomimetics, those systems that are of interest in this context are the systems which derive from natural starting materials such as amino acids, proteins and their derivatives.
The chemical linking of the siloxane structures and amino acid or protein structures, which are diametrically different as compounds, always poses a synthetic challenge. The task is therefore to overcome the difficulties presented by the differences in solubility behavior between siloxanes and amino acids. There has therefore been no lack of multifarious efforts to gain access to these interesting classes of substance, utilizing any of a very wide variety of chemical linking approaches.
For example, EP 1149855 describes one possible method for preparing arginine-functionalized siloxanes, using anhydride-functionalized siloxanes. The anhydride functionality is introduced, for example, by hydrosilylation of allylsuccinic anhydride, which is expensive and toxicologically objectionable. The siloxane is subsequently reacted with an excess of unprotected arginine in ethanol.
JP 2004-182680 describes a cosmetic product which comprises a silicone polymer modified by an amino acid derivative. In this case, a complex 4-stage synthesis is deployed, some intermediates of which are toxic, including a final step of coupling an isocyanate-containing siloxane with a modified amino acid. The end products, however, do not contain any free amino groups.
U.S. Pat. No. 5,516,869 discloses specific α,ω-amino acid-modified siloxanes which are synthesized by the hydrosilylating linking of alkenylpyrrolidones with α,ω-SiH-substituted siloxanes.
Another access point is the reaction, described in U.S. Pat. No. 5,412,074, of α,ω-epoxide-modified siloxanes with proteins, producing products whose siloxane units are connected to one another via polypeptide bridges of undefined length.
A similar access point is described by EP 1477512 A1, in which monoepoxysiloxanes are reacted with ε-polylysine. These structures have antimicrobial properties.
WO 2009084711 A1 describes the preparation of amino acid-modified siloxane emulsions. This publication gets around the divergent solubility properties of the epoxy-containing siloxanes and of the unprotected amino acids by performing the reaction in the presence of about 60% of water and 5% to 10% of emulsifiers. The emulsions have been tested in cosmetic applications.
The methods used in the prior art to couple amino acids or peptides to siloxanes involve reactions including epoxide ring-opening reactions, esterifications and transesterifications, amidations and substitution reactions. The disadvantages of the multi-stage processes described in the prior art include the use of toxic and difficult-to-manage raw materials, the use of expensive amino acid derivatives, the requirement for high temperatures, in some cases together with long reaction times, secondary reactions leading to discoloration and crosslinking, and low yields, as in reactions in emulsions, for example.